The present invention relates to encoding of signals and in particular, encoding of signals which are intended to be played back.
A number of systems have been proposed to enable monitoring of electronic media, including radio and television, to uniquely identify various signals. Examples of such prior art systems can be found in U.S. Pat. No. 3,004,104 Hembrooke, U.S. Pat. No. 4,040,099 Cook, U.S. Pat. No. 4,074,311 Tanner, U.S. Pat. No. 4,086,634 Cook, and U.S. Pat. No. 4,213,149 Janko.
These patents generally alter a recorded signal to prevent later unauthorized copying of it without affecting the signal when it is played back in the usual manner. In U.S. Pat. No. 3,004,104, there is disclosure of modifying a signal by varying a particular narrow frequency band for varying periods of time to introduce into the signal a particular code which uniquely identifies the product.
The addition of a particular signal to a recorded signal for the purpose of identifying the recorded signal or the selective deletion of the signal to allow identification of the signal is difficult and causes problems with respect to recognition of the particular recorded signal. These recorded signals are each unique and the signals must be modified in a manner that does not affect the intended quality of the signal. In addition to the huge number of recorded signals which require encoding, a problem occurs in being able to determine where the particular code starts and thus, to date, no systems have been effectively used in the marketplace to applicant's knowledge. With current technology, large computer memory banks may make some systems more feasible, however, there remains a need to provide a simple, efficient solution by which a signal may first be encoded and subsequently, when replayed, will allow convenient recognition without significantly affecting the quality of the signal and certainly without affecting the quality of the recorded signal in its entirety.